This invention relates to an imaging member (also referred herein as a photoreceptor) which includes a blocking layer containing an enriched concentration of nickel hydroxide. Such an imaging member is employed in for example an electrostatographic printing or copying apparatus.
There are numerous applications in the electrophotographic art wherein a coherent beam of radiation, typically from a helium-neon or diode laser is modulated by an input image data signal. The modulated beam is directed (scanned) across the surface of a photosensitive medium. The medium can be, for example, a photoreceptor drum or belt in a xerographic printer or copier, a photosensor CCD array, or a photosensitive film. Certain classes of photosensitive medium which can be characterized as "layered photoreceptors" have at least a partially transparent photosensitive layer overlying a conductive ground plane (which is part of a substrate). A problem inherent in using these layered photoreceptors, depending upon the physical characteristics, is the creation of dominant reflections of the incident coherent light on the surface of the photoreceptor which can give rise to optical interference effects. This condition is shown in FIG. 1 where coherent beams 1 and 2 are incident on a layered photoreceptor 6 comprising a charge transport layer 7, charge generating layer 8, and a ground plane 9. When the difference in the optical indices (i.e., the refractive index and the absorption constant) of the charge transport layer 7 and the charge generating layer 8 is large, one dominant reflection is at the interface between the charge transport layer 7 and the charge generating layer 8. There is a second dominant reflection from the top surface of layer 7. Depending on the optical path difference as determined by the thickness and index of refraction of layer 7, beams 1 and 2 can interfere constructively or destructively when they combine to form beam 3. When the additional optical path traveled by beam 1 (dashed rays) is an integer multiple of the wavelength of the light, constructive interference occurs, more light is reflected from the top of charge transport layer 7 and, hence, less light is absorbed by charge generating layer 8. Conversely, a path difference producing destructive interference means less light is lost out of the layer and more absorption occurs within the charge generating layer 8. The difference in absorption in the charge generating layer 8, typically due to layer thickness variations within the charge transport layer 7, is equivalent to a spatial variation in exposure on the surface. This spatial exposure variation present in the image formed on the photoreceptor becomes manifest in the output copy derived from the exposed photoreceptor. The pattern of light and dark interference fringes produced within a photoreceptor of the type shown in FIG. 1 when illuminated by for example a He-Ne laser with an output wavelength of 633 ran look like the grains on a sheet of plywood. Hence the term "plywood effect" is generically applied to this problem.
The conventional solutions to minimize the optical interference effects are problematic. For example, when the substrate surface is roughened to minimize the optical interference effects, the charge blocking layer typically has to be thick enough to completely cover (i.e., without bare patches) the substrate due to its surface roughness to prevent print defects caused by charge leakage from the substrate into the imaging member during the printing cycle. A relatively thick charge blocking layer which can be as thick as several microns may be undesirable in certain embodiments due to higher residual voltage.
There is a need, which the present invention addresses, for a charge blocking layer which can uniformly coat a substrate surface even at a relatively thin thickness and exhibits good charge blocking characteristics in a photoreceptor.
Conventional electrolytic techniques to form a metal oxide layer on a metal surface generally forms a mixture of a metal oxide and a metal hydroxide. However, it is believed that the amount of the metal hydroxide in the metal oxide layer resulting from the conventional techniques ranges from about 1% to about 5% (based on the number of the metal oxide and metal hydroxide molecules in the layer), the balance being the metal oxide.
The following documents disclose the use of a nickel oxide layer in an imaging member: Pinsler, U.S. Pat. No. 3,914,126, Pinsler, U.S. Pat. No. 3,907,650, Matyjakowski, U.S. Pat. No. 4,557,993, Leder, U.S. Pat. No. 4,013,463, and Ward et at., U.S. Pat. No. 4,098,655. In addition, Andrews et al., U.S. Pat. No. 5,215,853, discloses a layered photosensitive imaging member that is modified to minimize optical interference effects where the modification described is to form the ground plane surface by an electroforming process which leaves the surface with a black finish such as a black nickel layer.